1. Field of Invention
The present invention relates to a communications switch and more particularly to a thyristor switch for microwave signals.
2. Description of Related Art
(a) Thyristors
The name xe2x80x9cthyristorxe2x80x9d applies to a general family of semiconductor devices that exhibit bistable characteristics and that can be switched between a high-impedance, low-current OFF state and a low-impedance high-current ON state. Thyristors are well-known in the art. (See, for example, xe2x80x9cPhysics of Semiconductor Devicesxe2x80x9d, S. M. Sze, Wiley (1981); xe2x80x9cSemiconductor Power Devicesxe2x80x9d, S. Ghandhi, Wiley (1977).) Operationally, thyristors are analogous to bipolar transistors, in which both electrons and holes are involved in the transport process. The thyristor is a solid state semiconductor device usually made up of four layers with dopant sequence p-n-p-n, or to be more specific, p+-nxe2x88x92-p-n+, where the semiconductor material can be either Si (silicon) or GaAs (gallium arsenide) although most commercially made thyristors are constructed out of Si.
FIG. 1 shows a schematic of a representative two-terminal thyristor that is sometimes called a xe2x80x9cShockley Diode.xe2x80x9d For silicon devices, the typical doping of the four layers between an anode 2 and a cathode 4 is as follows: p+ (1019 cmxe2x88x923), nxe2x88x92 (1014 cmxe2x88x923), p (1016 cmxe2x88x923) and n+ (1019 cmxe2x88x923). This doping profile can be made by diffusion or by using epitaxial layers of the desired doping.
Another two terminal thyristor design used in the industry is a p+-p-nxe2x88x92-p-n+ structure as shown by the thyristor 5 in FIG. 2 where the doping profile 9 is also illustrated. This thyristor 5 consists of deep p type diffusions made simultaneously into either side of a slice of high resistivity nxe2x88x92 type silicon, with an alloyed or diffused n+ type region on one end to form the cathode 8. An aluminum layer is usually alloyed to the other end of the device to form a p+ type anode 6. Typically, thyristors are made from silicon and can be used for large power devices (e.g., 10 cmxc3x9710 cm). However, it is also possible to fabricate a thyristor out of GaAs using epitaxial layers as shown in FIG. 3.
In FIG. 3, the p+, nxe2x88x92, p and n+ semiconductor layers of the thyristor 10 are shown in a mesa-like structure with sloped walls disposed on a substrate 10. A metallic ohmic contact 12 to the p+ region serves as the anode. A metal air bridge 14 forms an ohmic contact to the n+ region and to a metallic ohmic contact 16 that serves as the cathode. The metal air bridge 14 can be fabricated by depositing photoresist, opening a via in the photoresist atop the n+ region, depositing metal through a mask, and dissolving the photoresist to leave the air bridge 14 as shown in FIG. 3. Alternatively, an air bridge design may include a dielectric material used for structural support.
A thyristor (e.g., FIGS. 1-3) has hysteresis or memory and is characterized by a high-resistance OFF state and a low-resistance ON state. FIG. 4 shows Va 18 as an operating voltage in the OFF state and Vc 22 as an operating voltage of the ON state. Transitions between the ON state and the OFF state are characterized by a break over voltage Vb 20 and a holding voltage Vh 26 as described in the following sequence.
The OFF state resistance is relatively high, and so the operating voltage Va 18 is essentially the applied voltage across the thyristor; that is, the resistance of the load has little effect. In the OFF state the current (I) is minimal.
When the thyristor is in the OFF state, a Turn-ON pulse voltage greater than the break over voltage Vb 20 causes the thyristor to transition to the ON state at the operating voltage Vc 22.
The operating voltage Vc 22 in the low-resistance ON state is less than the operating voltage Va 18 in the high-resistance OFF state, as characterized by a load line 24 that connects these operating points. The slope of the load line 24 is determined by the resistance of the load.
When the thyristor is in the ON state, A Turn-OFF pulse voltage less than the holding voltage Vh 26 causes the thyristor to transition to the OFF state at the operating voltage Va 18.
Repeat, etc
When the thyristor is in the OFF state, there is no transition when a pulse causes the voltage to decrease (e.g., below the holding voltage Vh); instead, the current continues to decrease along the continuous curve shown in FIG. 4. Similarly, when the thyristor is in the ON state, there is no transition when a pulse causes the voltage to increase; instead the current continues to increase along the continuous curve shown in FIG. 4.
Pulse circuits are typically used for operating the thyristor. Examples of a Turn-ON pulse 30 and a Turn-OFF pulse 32 are presented in FIG. 5 with reference to the thyristor I-V curve shown in FIG. 4. In the initial OFF state, the operating voltage is Va before the ON pulse 30 is applied. Because the amplitude Vg of the ON pulse 30 is greater than the break over voltage Vb, the thyristor switches from OFF to ON and the operating voltage drops to Vc. Similarly, in the initial ON state, the operating voltage is Vc before the OFF pulse 32 is applied. Because the amplitude (zero volts) of the OFF pulse 32 is less than the holding voltage Vh, the ON state collapses and the OFF state is obtained with the operating voltage Va.
The lightly doped nxe2x88x92 region shown in FIGS. 1-3 is critical to the operation of the thyristor. The thickness (sometimes called width) and the doping level of this nxe2x88x92 region both affect the voltage required to obtain reach through of the nxe2x88x92 region and therefore the magnitude of the break over voltage Vb.
Typically the application of thyristors has been mostly limited to applications such as power systems with relatively low frequencies (e.g., 60 Hz power control). Thyristors generally have not been used in applications involving higher frequencies including the range of microwaves (e.g., roughly 300 MHz-300 GHz).
(b) Telecommunications Switch Arrays
FIG. 6 illustrates a permutation switch element for use in the telecommunications industry. At each node there is the possibility of a connection between the input rows and the ouput columns. For example, Input r2 is connected to output s3 as shown in the diagram. There are N! different configurations possible in a permutation switch of dimension N (e.g., N=6 in FIG. 6). The important case where there are N inputs and N outputs is called an Nxc3x97N switch or an Nxc3x97N switch array, where an array may be made from a combination of switch elements.
A typical wavelength switch element used in the telecommunications industry is called an optical crossconnect switch (OXC). The OXC uses mirrors that can move a light spot from one location to another. The OXC is a permutation switch; that is, any one input is connected to only one output and vice versa. The net result is that the light intensity is retained during its passage through the switch and not diluted by a multiplicity of connecting paths.
A major disadvantage of the OXC is that it is not possible to vary the wavelength between input and output. That is, the wavelength of input r2 and output s3 must be the same. Many optical networks require the additional flexibility of assigning to the output s3 a wavelength different from that of the input r2. This can be done in the network by adding much more complex and costly extra equipment that effectively adds considerable cost to the OXC.
In FIG. 6, the array size is drawn for N=6. However, the array size for a crossconnect application should be appreciably larger, perhaps large enough to accommodate xcx9c50 fibers in each cable and xcx9c20 wavelengths in each fiber. A typical crossconnect switch can therefore have N xcx9c1,000 to best optimize the performance of the communication network.
It is possible to use tiling to assemble a multiplicity of smaller mxc3x97m crossconnect arrays into a larger Nxc3x97N array as shown in FIG. 7. The system of 9 arrays or chips is shown within the bold line. All interconnections can be made on a printed circuit board and carry the full bitrate. For example 100 68xc3x9768 chips can be arranged to form a larger array of 10*68xc3x9710*68=680xc3x97680. Tiling obviously requires appreciable cost, especially at the higher bitrates and larger array sizes.
Alternative approaches to optical switching devices may include conversion of an optical signal to an electrical signal that can be manipulated using digital switching devices and then converted back to an optical signal. For example, a digital optical signal with bitrate B can be passed through a photodetector, in which case it is converted to an electronic signal with the same bitrate. The bit rate B of information flow in each optical stream at each wavelength can be any one of the standard values. For example, B=2.5, 10, and 40 Gbps, for the industry standards OC-8, OC-192 and OC-768, respectively. The general trend in optical communications is for the higher bit rates.
For switching electrical signals, digital switches are often used to create crossconnect arrays with a structure similar to the switch shown in FIG. 6. A digital switch can be located at each node of FIG. 6. Digital switch arrays are composed of active digital switches that operate at the bitrate B. Each switch senses the digital electrical signal at the switch input and recreates the digital electrical signal at the switch output. The switches require power and this power increases with the bitrate. The switch operation is done electrically at microwave or millimeter wave frequencies. For example, at a bitrate of B=10 Gbps, the switch time to go from a xe2x80x9c1xe2x80x9d to a xe2x80x9c0xe2x80x9d is less than 1/B or less than 0.1 nanosecond. This is in contrast with the array switching time which is about 1 microsecond.
Digital switches convert each incoming digital stream of 0""s and 1""s into another digital stream with the same amplitude and waveform shape. The digital switches are totally active and respond to the actual bit rate. For example, a switch which is designed for bitrate B=10 Gbps must actively respond to this data rate. The time for this active switching operation is of the order of 1/B, which for this example is 0.1 nanosecond. Also, these chips can be used in more generalized configurations than the simple permutation configuration shown in FIG. 6. With digital switches, one input can be sent to two or more outputs although this functionality is generally not critical for applications involving system reconfiguration and wavelength modification for optimal system utilization and protection.
In general, the array switching time required to reconfigure a switch array in order to change the linkages and wavelengths need not be less than 1 ms., which is an acceptably small fraction of the xcx9c50 ms time required for setup and confirming communication between linkages xcx9c100 km apart. Therefore, the ability of digital switches to change configurations in substantially less than one millisecond is generally not relevant in most telecommunications applications.
Digital switch arrays are characterized by their array size N and their bitrate B. Typically, a given array configuration of N inputs and N outputs can be switched to another configuration having the inputs and outputs arranged in a different order within a time period of about one microsecond. Some nominal values of B and N corresponding to known discrete components are given in FIG. 8, where the optimal values of the data points take the general shape of a hyperbola.
These chips can be made of GaAs as on the left side of FIG. 8 or Si as on the right side of FIG. 8. Other materials are also possible. Typically large arrays have low bitrates and vice versa because of issues related to power consumption and bit rate for these active devices. The chips represented in FIG. 8 lie on or to the left of the characteristic hyperbola. However, the region to the right of the hyperbola with a relatively high bit rate and large array size is a more desirable operating region for many telecommunications applications and so the applicability of these devices is limited.
Accordingly, it is an object of this invention to provide a thyristor for switching microwave signals.
It is a further object to provide a thyristor that reliably transmits microwave signals.
It is a further object to provide a microwave switch that enables microwave transmission and switch control along a single line.
In a preferred embodiment of the present invention, a thyristor for switching microwave signals includes semiconductor layers disposed on a substrate. A first surface of the thyristor defines an anode, and a second surface of the thyristor defines a cathode. The semiconductor layers include at least one semi-insulating layer. The thyristor transmits a microwave signal between the anode and the cathode in an ON state and blocks the microwave signal between the anode and the cathode in an OFF state.
Preferably there are two semi-insulating layers. In a preferred six-layer configuration, the thyristor may be configured as either p+-i-n-i-p-n+ or n+-i-p-i-n-p+, where xe2x80x9cixe2x80x9d denotes a semi-insulating layer.
The layers may be made from either GaAs (gallium arsenide) or Si (silicon). In the case where the layers are made from GaAs, the semi-insulating layers may include semi-insulating GaAs, and in the case where the layers are made from Si the semi-insulating layers may include intrinsic Si. The thyristor may be configured as a mesa with an air bridge as part of the metallic connections at the anode and cathode.